But UK experts said the idea of obesity as an infectious illness was unlikely.

The properties of adenovirus-36, long known as a cause of colds and eye infections, have been under investigation for several years, with animal experiments suggesting that it might have some connection to the laying down of fat cells.

This study provides stronger evidence that some obesity cases may involve viral infections

Dr Magdalena Pasarica, Pennington Center

The idea that a virus could be a factor in some cases of obesity has not found widespread support, with most insisting that weight gain can be explained far more simply through poor diet and lack of exercise.

The Pennington team's latest research uses human stem cells in the laboratory.

Stem cells are the body's "master cells", and scientists have been gradually unlocking the secrets of how they are transformed to make new tissue.

The researchers extracted stem cells from fatty tissue taken from liposuction patients, and exposed some to adenovirus-36.

After the cells were left for a week, those exposed to the virus had changed into human fat cells, while those left to grow without the virus had not.

Treatment hope

While the scientists say that they are unsure how the virus might cause obesity in humans, they believe that they have found a gene within it that causes fat accumulation in animals.

I just can't see how this explains the epidemic of obesity we are experiencing

Dr Nick Finer, Centre for Obesity Research

Dr Magdalena Pasarica, who led the research, said: "We're not saying that a virus is the only cause of obesity, but this study provides stronger evidence that some obesity cases may involve viral infections.

"Not all infected people will develop obesity - we would ultimately like to identify the underlying factors that predispose some obese people to develop this virus and eventually find a way to treat it."

Not everyone is convinced, however. Dr Colin Waine, of the National Obesity Forum in the UK, said that while it was attractive to chase a "holy grail", more practical measures were the best way to tackle the western world's obesity epidemic.

Dr Nick Finer, from the Centre for Obesity Research, said that while the virus was "interesting", the idea of an infectious cause of obesity was hard to accept ahead of far more convincing explanations.

"I just can't see how this explains the epidemic of obesity we are experiencing," he said.